composite
may be lower than the ceramics, the polymer matrix
largely increased the flexibility of the devices.
For VINGs, the device fabrication was largely limited by
the assembly process of the nanowire arrays.Therefore, most
of the investigation of VINGs was focused on ZnO nanowire
arrays, while LINGs were also suffering from the same
problem. However, the nanowire-composite can be easily
obtained by directly mixing the as-synthesized nanowires
with the polymer matrix such as the PDMS. Therefore, it
was a general structure for building nanogenerators. Table 2
listed the reported nanowire-composite nanogenerators with
different piezoelectric materials. For example, Hu and coauthors
have demonstrated the high output nanogenerators by
rational unipolar assembly of conical ZnO nanowires [49].
The macroscopic piezoelectric potential across the thickness
of the nanogenerator with output voltage up to 2V can be
produced under a compressive strain of 0.11% at a strain rate
of 3.67%/s. A similar structure was also reported by Jung et
al. by using NaNbO3 and GaN nanowires composited with
PDMS between a pair of electrodes, respectively [9, 50]. The
output voltage and current density of the NaNbO3-based
device are up to 3.2Vand 72 nA under a compressive strain of
0.23%(Figure 12). Recently, Xu et al. have reported the PMNPT
nanowire-based nanocomposites and nanogenerators
[55]. The output voltage and current density of this PMN-PT
composite nanogenerator are up to 7.8V and 4.58